The present invention relates to a method for efficiently extracting an amino acid ester in free base form from a hydrous solution containing a mineral acid salt thereof.
Amino acid esters are important as an intermediate used in peptide synthesis. Methyl L-phenylalaninate, in particular, have recently drawn attention as a main raw material for Aspartame, an artificial sweetening.
For the esterification of an amino acid, ordinarily hydrogen chloride gas or concentrated sulfuric acid is introduced into an alcohol suspension of the amino acid.
The thus-produced amino acid ester is typically isolated in the form of a salt, such as the hydrochloride or the sulfate. When it is desired to use the amino acid ester as a reactant, e.g., in peptide synthesis, the salt is made into an aqueous solution, a base is added to the aqueous solution to convert the amino acid ester to its free base form, which is then extracted with an organic solvent, and the resulting organic solvent solution of the free base form is used per se in peptide synthesis. The reason for this is that amino acid esters tend to cause self-condensation to form a polypeptide or a diketo-piperazine derivative and accordingly have a problem in stability while their mineral acid salts have excellent storage stability.
When a free amino acid ester is needed, the above practice of isolating it in the form of a mineral acid salt is not advantageous from the viewpoints of the yield and operation.
Japanese Patent Publication No. 267600/1986 discloses a process for extracting free L-phenylalanine methyl ester with a water-immiscible organic solvent, after L-phenylalanine has been esterified with methanol in the presence of a strong acid as a catalyst followed by neutralizing the strong acid catalyst with base. Page 2, right lower column, lines 2-5 of that reference states that it is sufficient to use a molar amount of the base which is equal to or greater than the acid (such as sulfuric acid or hydrochloric acid) used as the catalyst. In general, a molar amount equal to or up to twice that of the strong acid is used, since an excessive base would hydrolyze the methyl ester. No specific pH value is disclosed for the neutralization procedure. However, the neutralization procedure of the reference is used to eliminate the strong acid used as the catalyst. It is not used to arrive at a neutral reaction solution.
Thus, after the neutralization procedure, the acid used as the catalyst is completely neutralized and, at the same time, the L-phenylalanine methyl ester in the solution is converted completely to its free base.
Because L-phenylalanine methyl ester is itself a base, after the neutralization of the strong acid, the solution will have a pH value much higher than the neutral value of pH 7, even if an amount of base in excess over the amount of the corresponding strong acid is not used.
In both of the above cases of (1) isolating a mineral acid salt of an amino acid ester, neutralizing the salt in its aqueous solution and extracting an amino acid ester and (2) esterifying an amino acid in the presence of an acid and subjecting the reaction mixture to neutralization and extraction to obtain an organic solvent solution containing an amino acid ester, there is a problem of hydrolysis of amino acid ester during neutralization and extraction. Heretofore, no adequate solution to this problem has existed.
The primary object of this invention is to provide an improved method for extracting an amino acid ester in free base form from a hydrous solution of a mineral acid salt of the amino acid ester.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.